Stepped Fabry-Perot etalons are typically comprised of a body of transparent material having a top planar surface and two bottom planar surfaces that are parallel to the top surface but are positioned at different distances from the top surface so that there is a step between the two bottom surfaces. The bottom surface that is closer to the top surface is commonly referred to as the thin side and the bottom surface that is farther from the top surface is commonly referred to as the thick side. In determining the frequency of a beam of light projected onto the top surface, use is made of the transmittance functions of the top and bottom surfaces of the etalon.
In conventional etalons, the step between the two bottom surfaces is perpendicular to the surfaces, and reflections from the step back into the etalon are at a different angle than that of the original beam of light. Since the transmittance function of the etalon depends on the angle at which light impinges on the top and bottom surfaces of the etalon, the transmittance function for the original beam of light and the transmittance function for the light reflected from the step are different. Thus, the overall transmittance function is defined by the uncontrolled interference of the original and reflected light. The etalon characteristic of the stepped etalon becomes irregular. This effect occurs in the proximity of the step so as to make the area near the step unusable. This is an obstacle to the miniaturization of the etalon.
In an attempt to reduce this deleterious effect, random features have been formed on the step to scatter the light. Doing so helps to a degree because the more light scattered, the less light is reflected, but it does not solve the problem completely.